Make ERC use completion-at-point

Make ERC use completion-at-point
* lisp/erc/erc.el (erc-mode-map): Use completion-at-point.
(erc-mode): Tell completion-at-point to obey erc-complete-functions.
(erc-complete-word-at-point): New function.
(erc-complete-word): Make it obsolete.
* lisp/erc/erc-pcomplete.el (erc-pcompletions-at-point): New function.
(pcomplete): Use it.
* lisp/erc/erc-dcc.el (erc-dcc-chat-mode-map): Use completion-at-point.
(erc-dcc-chat-mode): Tell completion-at-point to obey
erc-complete-functions.
* lisp/erc/erc-button.el (erc-button-next-function): New function extracted from
erc-button-next.
(button, erc-button-next): Use it.